A method of forming a symmetrical grinding wheel

ABSTRACT

An abrasive grinding wheel is formed by compression of a mix including abrasive and resin particles, and includes the steps: A. FILLING THE MIX INTO AN ANNULAR RECESS FORMED BY A SLEEVE, A STRIPPER PLATEN RECEIVED AXIALLY INTO THE SLEEVE, AND A CENTRAL STEM PROJECTING AXIALLY WITHIN THE SLEEVE; AND B. ADVANCING A PLUNGER RELATIVELY TOWARD THE PLATEN TO COMPRESS THE MIX IN THE RECESS AND TO AN EXTENT THAT THE MIX FRICTIONALLY ENGAGES THE SLEEVE INNER WALL, THE SLEEVE BEING DISPLACED AXIALLY AND RELATIVELY TOWARD THE PLATEN DURING THE MIX COMPRESSION AND TO AN EXTENT CAUSING GENERALLY EQUALIZED COMPRESSION PACKING OF THE MIX AT AXIALLY OPPOSITE SIDE REGIONS OF THE MIX.

United States Patent Whitaker et al. NOV. 20, 1973 [54] A METHOD OFFORMING A 3,208,836 9/1965 Biglin et a1 51/298 SYMMETRICAL GRINDINGWHEEL 2,939,777 6/1960 Gregor et a1.... 51/298 3,273,984 9/1966 Nelson51/298 Inventors: Noble D. Whitaker, Los Alamitos;

Thomas M. Akashi, Gardena; John A. Aurentz, Palos Verdes Peninsula,

all of Calif.

[73] Assignee: Pacific Grinding Wheel Co., Inc.,

Marysville, Wash.

[22] Filed: Nov. 9, 1970 [21] Appl. No.: 87,710

[52] [1.5. CI 51/295, 51/298, 264/331 [51] Int. Cl B24d 3/30 [58] Fieldof Search 51/295, 298, 309,

[56] References Cited UNITED STATES PATENTS 3,369,879 2/1968 Miller51/293 3,615,302 10/1971 Rowse 51/309 3,573,013 3/1971 Cum et a1. 51/3092,943,926 7/1960 Goepfert 51/298 3,576,610 4/1971 Mathewson 51/295Primary ExaminerD0nald J. Arnold Attorney-White, Haefliger and Bachand[57] ABSTRACT An abrasive grinding wheel is formed by compression of amix including abrasive and resin particles, and includes the steps:

a. filling the mix into an annular recess formed by a sleeve, a stripperplaten received axially into the sleeve, and a central stem projectingaxially within the sleeve; and

b. advancing a plunger relatively toward the platen to compress the mixin the recess and to an extent that the mix frictionally engages thesleeve inner wall, the sleeve being displaced axially and relativelytoward the platen during the mix compression and to an extent causinggenerally equalized compression packing of the mix at axially oppositeside regions of the mix.

2 Claims, 4 Drawing Figures 1 A METHOD OF FORMING A SYMMETRICAL GRINDINGWHEEL BACKGROUND OF THE INVENTION This invention relates generally totheforming of abrasivegrinding wheels, and more particularly concerns'apparatus for'molding a mix containing abrasive and resin particles toproduce such wheels.

In producing resin bonded grinding wheels, it is usual ""to mold theabrasive, and resin particles into final shape by applying pressure and,thereafter, curing the resin outside the mold. This'procedure makes italmost impossible to obtain a wheel having accurate dimensions,

the long cure cycle after molding frequently causing warping. Inco-pending U. S. Pat. application Ser. No. 880,949 now US. Pat; No.3,66l ,544 there is disclosed a method of molding abrasive and resinparticles to en- I able curing of the resin in the mold; enabling rapidpro- SUMMARY OF THE INVENTION It is a major object of the invention toprovide apparatus and method for overcoming 'theabove problems and toenable rapid formation of-a resin bonded grinding wheel of high qualityhaving particle density or compaction characteristics which are the sameat opposite side regions of the wheel; ie are symmetricalwith respect toa plane normal to the wheel axis and bisecting the wheel material.

Basically, the molding apparatus comprises a mold sleeve defining acentral axis; a mold platen received axially into the sleeve, the sleevemounted for axial displacement relative to the platen; a central stemprojecting axially within the sleeve so that the sleeve, platen and stemform an annular recess to receive the mix for compression; and anannular plunger movable axially in the sleeve relatively toward theplaten and relative to the sleeve, thereby to compress the mix causingit to frictionally displace the sleeve axially relatively toward theplaten to an extent such that the platen and plunger effect symmetricalcompression of the mix at opposite sides of a plane normal to the axisand bisecting the compressed mix. Typically, the stem may be mounted foraxial displacement relative to the platen, and the compression of themix causes it to frictionally displace the stem axially relativelytoward the platen, to enhance the symmetrical compression as referredto. Accordingly, the'resultant wheel will grind and abrade at the samerate at opposite side regions thereof.

Additional objects and advantages of the invention includes theprovision of a compression spring or springs located to yieldably resistsleeve axial displacement-relatively toward the platen, and also toresist stem displacement as described; means to heat the platen andplunger for rapid'heat transfer into the mix during compression in themold; means to introduce the mix into the recess with the plunger inretracted position; and means rotatable within the mix receiving recessto substantially uniformly distribute the granular mix thereon at timeswhen the plunger is retracted.

In its method aspects, the basic steps of the invention include fillingthe mix into the recess formed by the sleeve, platen and control stem;advancing the plunger relatively toward the platen to compress the mixin the recess and to an extent that the mix frictionally engages thesleeve inner wall or bore; and effecting sleeve displacement axiallyandrelatively toward the platen during the mix compression and to an extentcausing generally equalized compression packing or dens'ification of themolded wheel at opposite side regions thereof, and during curing in themold. I

These and other objects and advantages of the invention, as well as thedetails of an illustrative embodiment, will be more fully understoodfrom the following description and drawings, in which:

DRAWING DESCRIPTION FIG. 1 is an exploded perspective showing of fillingapparatus;

FIG. 2 is a side elevation taken insection to show the FIG. 1 apparatusduring a mold filling operation;

FIG. 3 is a side elevation taken in section to show the FIG. 1 apparatusfunctioningas a spreader; and

FIG. 4 is a side elevation taken in section to show molding of a mixfilled into and spread in a mold.

DETAILED DESCRIPTION In the drawings, a cylindrical mold platen orstripper plate 10 is partially received upwardly into a mold sleeve 11which is mounted for axial displacement relative to the platen. Suchmounting may with unusual advantage include a series of compressionsprings 12 extending about guide pins 13 spaced about the sleeve, sothat the spring upper terminals support a sleeve flange 11a. The lowerterminals of the springs rest on a slide plate 14 which is movablelaterally in the direction of arrows 15. That plate also supports theplaten 10.

A central stem 16 projects axially within the sleeve so that the sleeve,platen l0 and stem 16 form an annular recess 17 to receive the mixgenerally indicated at 18. In general, the mix may be formed by mixingan abrasive composition containing abrasive particles and preferably,friable particles, with a liquid, multi-stage, thermosetting resin andcoating substantially all of the individual particles with the resin;curing the resin to the B-stage while the resin is in contact with theparticles to form a fusible solid coating on the particles and formingindividual particles coated with the B-stage resin. Such mix formationis described in detail'in copending application of Noble D. Whitaker,Ser. No. 880,949, filed Nov. 28, 1969 now US Pat. No. 3,661,544.

Referring now to FIGS. l-3, means is there shown to introduce the mix 18into the recess 17, and to uniformly distribute the mix therein, attimes when a mold plunger, tobe described, is retracted from the recess.Such means may advantageously include a conical hopper 20 containing mixparticles 18a and having a bottom opening 21; and a spreader cone 22having an upposition as seen in FIG. 3 in which it closes the opening21, and a down-position as seen in FIG. 2 in which it allows mixparticles to gravitate through the opening 21 and onto the spreader conefor lateral spreading and free fall into the recess 17.

A ring 23 is concentrically connected with the spreader cone 22 as byspider arms 24 which extend radially and with spacing therebetween topass the mix particles, the ring preventing lateral escape of fallingparticles. Pins 25 freely hang or dangle from the arms 24 and dependinto the mix 18. Upon manual rotation of the ring, as facilitated byvertical axle 27 centrally connected with the spreader and seated on thestem 16, the lower tips of the pins drag through the mix and distributeit evenly in the recess 17 in preparation for subsequent molding.

Note in FIG. 1 the support stand 28 for the hopper funnel 20, with legs29 adapted to seat on sleeve 11, at which time the axle 27 projectsupwardly through the funnel and into bearing 30 carried by arms 31attached to the stand 28 as shown.

When filling of mix into recess 17 is complete, the hopper 20, stand 28,spreader 22, ring 23, and pins 25 are lifted away from the sleeve 1 l,leaving it openly exposed. Next the slide plate 14 is shifted laterallyand sufficiently to bring the recess 17 and mix 18 directly under aplunger 33 at a molding station. For this purpose the slide plate may besupported on a fixed base plate 34. Pins 35 and 36 project in openings38 and 39 in the base plate, and in common opening 37 in the slideplate, to locate the latter in filling and molding position as seen inFIGS. 3 and 4, respectively.

In accordance with an important aspect of the invention, the plunger 33is formed as an annulus, and movable vertically downwardly and axiallyin the sleeve 11 relatively toward the stripper platen and to thesleeve, thereby to compress the mix 18. The operation is such as tocause the mix to transmit force radially outwardly, to frictionallydisplace the sleeve axially relatively toward the platen, i.e.,downwardly, whereby the platen and plunger effect symmetricalcompression of the mix at opposite sides of a plane 40 normal to centralaxis 41 and bisecting the mix. In effect, the mix is treated as thoughplungers were displaced thereinto from opposite sides; however, only oneplunger is needed, saving in cost of the equipment.

Note in this regard that this treatment is further effected throughmounting of the stem 16 for axial displacement relative to the platen,the radially inward compression of the mix causing it to frictionallydisplace the stem downwardly along with the sleeve. Platen remainsfixed, at all times.

Springs 12 yieldably resist downward displacement of the sleeve, and acentral spring 42 likewise yieldably resists downward displacement ofthe stem. The top of the mix is compressed toward the platen by thedistance d during such molding action. In addition, heat is transferredinto the mix from the platen and plunger by conduction from heaters 43in base plate 34, and heaters 44 in top plate 45 supporting the plunger33. Heat and pressure applied to the mix forms a coherent mass ofparticles which is cured from B-stage in a few min- -utes. Pressures maybe between 800 and 1,200 psi, and

temperatures of the compressed mix between 300 and 350 F, the cure beingcomplete in less than 5 minutes. The resultant product comprises agrinding wheel of excellent quality.

Finally, the plunger 33 may be withdrawn to provide access to thefinished product, for removal from the mold, and the slide platereturned to FIG. 2-3 position. In this regard, the sleeve 16 and pins 13can be lifted off the slide plate to facilitate wheel removal.

Stem 16 is guided for vertical movement by a central pin 50 attached at51 to the slide plate. A head 52 on the pin limits upward displacementof the stern as urged by the spring 42.

EXAMPLE A pre-mix is made as follows:

One hundred grams of epoxy resin (Epon 828), 20 grams of a modifiedamine as curing agent (Epon curing agent Z), and grams of acetone as adiluent to lower the viscosity of the epoxy resin are mixed to-, getherat room temperature and, thereafter, there is added thereto a mixturecomposed of 1,000 grams of 46 grit A1 0 15 grams of expanded perlite Omesh and 2 grams of chrome yellow pigment.

As is known in the art, Epon 828 is a diglycidyl ether of bisphenol Aand is a liquid at room temperature having a viscosity at 25C. ofbetween and poises, and epoxy equivalent of to 210, a specific gravityat 20C. of 1.167, and an equivalent weight of 85. Epon curing agent Z isa liquid aromatic amine which when used with epoxy resins yieldscrosslinked polymers. Curing agent Z has a viscosity of about 2,000centipoises at 25C.

The thus formed mixture of epoxy resin, curing agent and abrasive isblended thoroughly and spread on baking trays approximately one-halfinch thick, and placed in an air circulating oven at to 200F. for 45 to60 minutes. During this period the mixture may be periodically agitatedto prevent caking.

After 60 minutes the material is removed from the oven and allowed tocool to room temperature. When the material is first removed from theoven the individual particles of abrasive (A1 0 the perlite (forporosity) and the pigment are all coated with a tacky and soft coating.As the material cools the coating becomes very dry and brittle. Duringthe cooling period the particles are agitated or fluffed" in order toprevent cak ing. If desired the resultant individual particles or powdercan be screened to remove agglomerates created during the processing orcan be milled or ground to produce a relatively uniform particle size.Care should be taken during grinding not to remove or abrade the resingcoating.

The thus formed coated abrasive particles and perlite can be stored formany months prior to molding the particles into an abrasive tool.

When it is desired to make a grinding wheel, the premix, made asindicated above, is placed into a mold as described. In the presentexample, the pre-mix was placed in a 6 inch mold preheated to 325F. Themix is then compressed at 1,000 psi as described and maintained at thistemperature and pressure for 5 minutes. The wheel was then removed fromthe mold, cooled to room temperature and immediately placed on a surfacegrinder. The wheel removed 60 grams of tool steel (R 42-46) with a wheelweight loss of2 grams. No coolant was used. The metal grinding chipsremoved were of classical spiral machined chip configuration indicatingideal sharpness.

Referring back to FIG. 4, it will be noted that there is slightdownwardly tapered annular clearance at 60 between the bore 11a and thesleeve or case 11 and the outer annular surfaces 10a and 33a of thestripper platen and plunger 33. For this purpose, surfaces 10a and 33amay be cylindrical about axis 41, and bore 1 la may have vertical taper,as shown. For best results, the taper angularity at 60 should be between4 and 35 minutes (which, in the event surface 11a is tapered, asdescribed, is between 0.001 and 0.010 inches per inch taper):

Also, there is slight downwardly tapered annular clearance at 61 betweenthe outer surface 16a of the stem 16 and the bores 33b and b of theplunger 33 and stripper platen 10 respectively. For this purpose, thosebores may be cylindrical about axis 41, and the annular stem surface 16amay taper upwardly, as shown. Again, for best results as referred toabove, the taper angularity at 61 should lie between 2 and 17.5 minutes(which, in the event surface 16a is tapered, as described, is between0.00058 and 0.005 inches in a radial direction per inch of length in theaxial direction.) Any further reduction in taper angularity below thelower limits referred to will result in excessive steel mold wear,excessive ejection pressure required to free the molded product, andexcessive abrasion of that product during ejection. Any further increasein taper angularity above the upper limits referred to will result information of excessive flash at the periphery of the product wheel andloss of dimensional integrity.

1 claim:

1. In the method of forming an abrasive grinding wheel from an abrasivemix composition consisting of I abrasive particles coated withthermosetting resin cured to B-stage, said method involving the use of amold sleeve defining an axis, a mold platen received axially into thesleeve, the sleeve mounted for axial displacement relative to theplaten, a central stem projecting axially within the sleeve so that thesleeve, platen sleeve, relative to the stem and relatively toward theplaten to compress the mix in the recess and to an extent that the mixfrictionally engages the sleeve inner wall and stem outer wall, and c.effecting sleeve and stem displacement axially and relatively toward theplaten during said compression of the mix and in response to forcetransmission from the mix to the sleeve and stem via the mix inner andouter annular peripheries and to an extent causing generally equalizedcompression packing of the mix at axially opposite side regions of themix, and yieldably resisting said sleeve and stem displacement. 2. Themethod of claim- 1 including transferring heat into the compressed mix,and continuing said heat transfer and compression to effect curing ofthe resin in said recess.

2. The method of claim 1 including transferring heat into the compressedmix, and continuing said heat transfer and compression to effect curingof the resin in said recess.